1. Technical Field
The present invention pertains to methods and apparatus for imparting low amplitude vibrations to hard tissue such as bone. The invention has utility in hearing aids, assistive listening devices, bone growth stimulation, various therapeutic applications, and the like.
2. Discussion of the Prior Art
It is well known that imparting acoustic frequency vibrations to the human skull, either directly or via teeth, results in improved hearing in certain hearing impaired individuals. Hearing aids and assistive listening devices taking advantage of this phenomenon generally include a microphone for transducing ambient acoustic energy into an electrical signal, an audio amplifier, a transducer for converting the amplified audio signal to mechanical vibrations, and some means of imparting the vibrations to a tooth or to bone structure in the skull. The vibrations ultimately stimulate the cochlea, resulting in a perception of sound. Examples of such devices are disclosed in U.S. Pat. Nos. 2,161,169 (Jefferis), 2,995,633 (Puharich), 3,594,514 (Wingrove), 4,498,461 (Hakansson), 4,606,329 (Hough), 4,612,915 (Hough), 4,774,933 (Hough) and 5,033,999 (Mersky). Ultrasonic vibratory transducers have been used to provide vibrations conducted via bone to organs other than the cochlea (e.g., the vestibular saccule) to effect sound perception in humans. An example of this approach is found in U.S. Pat. No. 4,982,434 (Lenhardt et al).
It is also known that mechanical stresses (e.g., in the form of vibrations) applied to bone tissue generate electrical potentials across the tissue, much like a piezoelectric effect. Further, it is known that when an elongate bone is subjected to a compressive force applied in the direction of its longitudinal axis, the periosteum is induced to proliferate and form new bone tissue. It is believed that this phenomenon is related to the electrical energy produced in the bone by its inherent piezoelectricity. It is also known that this phenomenon operates in a reverse manner, whereby application of alternating electrical current to living bone can produce mechanical deformation of the bone tissue.
Bone growth stimulators are disclosed in U.S. Pat. Nos. 4,314,554 (Greatbatch), 4,467,809 (Brighton) and 4,665,920 (Campbell) and typically utilize electrical current flow through bone tissue to effect bone tissue growth in the healing of fractures, for example. Although not so stated in these patents, it is postulated herein that the disclosed bone growth simulators actually induce vibrations in the bone tissue. Whether or not this is correct, these patents demonstrate the effectiveness of electrical stimulation of bone and suggest that direct application of vibrations to bone tissue produces electrical currents in the tissue, ostensibly via strain of the bone tissue and the resultant piezoelectric effect, to result in fracture healing and other therapeutic effects.
One of the problems associated with prior art attempts to apply vibrations to bones relates to the effectiveness of the vibrator itself to accurately transduce the applied electrical signals into mechanical vibrations. Another problem relates to the manner in which the transducer is coupled to the hard bone tissue. In particular, prior art coupling techniques have the disadvantages of: low coupling efficiency (i.e., significant loss of mechanical energy); deterioration of coupling efficiency over time; difficulty of removing or replacing the vibrating member; and a combination of these disadvantages. For example, reference is made to the aforementioned Hough patents and their disclosures of securing a bone vibrator to the skull either by bone screws, adhesive, or the like. These implanted vibrators respond to supracutaneously mounted electromagnetic transmitters delivering electromagnetic signals transcutaneously to the implanted vibrators. This transcutaneous, as opposed to direct, coupling of the signal to the vibrator results in considerable loss of energy since the energy loss between the transmitter and vibrator increases with the square of the distance between them. Moreover, the magnetic attraction between the transmitter and vibrator deteriorates over time and results in further loss of efficiency. As a practical manner, then, the implanted vibrator and external transmitter must be separated by only a thin layer of skin, and this layer must be so compressed between the members as to often result in pain for the wearer. Moreover, the compression of the skin results in edema, thereby further separating the transmitter and the vibrator and causing a further loss of transmission efficiency.
Many of the patents listed above disclose vibrating members relying on osseointegration to eventually secure the members to bone tissue. As a result, removal of the vibrator for replacement or for other reasons involves a major surgical procedure. Further, since osseointegration can only take place over time, such devices are impractical for use in fracture repair or other applications requiring immediate treatment.
Prior vibrators employed to impart vibrations to bone tissue are typically magnetic or piezoelectric. Magnetic transducers involve reciprocating translation of a magnetically permeable rod or similar armature member. These devices tend to be inefficient in transducing electrical energy into reciprocating translatory motion, and are operable only over limited frequency ranges due to inertial constraints of the movable member. Piezo-ceramic devices also tend to be inefficient in that they require relatively high voltages, and are notoriously ineffective at frequency ranges below 1 KHz.